1. Field of the Invention
This invention relates to a torque sensor for detecting the torque of a rotating shaft, more particularly to a magnetostriction type torque sensor for measuring the torque of a drive shaft, steering shaft or other rotating shaft of an automobile and the like, and to a method of manufacturing the same.
2. Description of the Prior Art
The indirect type torque sensor which measures the torque of a shaft by sensing the amount of twist therein is unable to measure static torque. Because of this inadequacy, there have recently been introduced a number of direct type torque sensors operating on the principle of magnetostriction. As an example of such a torque sensor there can be mentioned the one described in Japanese laid-open Patent Publication No. 57(1982)-211030, wherein a ribbon-like magnetostrictive strip is wound on a shaft whose torque is to be measured. A similar torque sensor was also disclosed in Japanese laid-open Patent Publication No. 59(1984)-166828.
The structure of the torque sensor disclosed by this publication requires that the magnetostrictive member (magnetic member) be fixed directly on the shaft whose torque is to be measured so that the shaft itself becomes one component of the torque sensor. This is disadvantageous for several reasons. First, during manufacture of the torque sensor, it is generally necessary to attach the magnetostrictive member to a shaft of considerable length such as an automobile drive shaft, and this is difficult to do with high positional precision. Then, after the magnetostrictive member has been fixed on the shaft and up to the time that the shaft is installed in the vehicle, which is generally late in the assembly process, it is necessary to take great care in transporting and storing the shaft bearing the magnetostrictive member so as to protect the member from damage and adherence of dust or the like. The need to take these precautions greatly complicates the overall process of shaft installation.
Moreover, since the shaft whose torque is to be measured is involved as one component of the sensor, the sensor cannot be completed without mounting the other components on the shaft. As a result, it is not possible to adjust the gap between the magnetostrictive member and the associated coils until the assembly is carried out. Another disadvantage arises from the fact that drive shafts and other such automotive parts are only required to have adequate strength and are not required to have high dimensional precision. It therefore becomes necessary to use a special, separate adjustment means for adjusting the gap, which leads to further inconveniences as regards inventory control, performance control, maintenance and the like.
Also, since the structure is such that the coil and other components which are relatively susceptible to damage by mechanical shock are not capable of being easily removed form the exterior, special care has to be exercised during assembly and installation. This structure is also disadvantageous from the point of maintenance.
The conventional magnetostrictive type torque sensor has further required that a magnetic material exhibiting magnetostriction (e.g. a magnetic amorphous film) be attached to the outer periphery of the torque transmission member or shaft. In this case, if a large gap is present between the torque transmission member and the magnetic member attached thereto, an error is apt to arise in the measurement because of slippage between these two members, while also disadvantageously cracks and other forms of physical degradation are likely to occur with prolonged use, thus shortening the service life of the sensor. It has therefore been necessary to make every effort to minimize the size of the gap between the torque transmission member and the magnetic member at the time of attachment. Methods aimed at achieving this are disclosed in Japanese laid-open Patent Publication 57(1982)-2110930 and elsewhere and include a method of mold-bonding the entire magnetic member using a synthetic resin bonding agent, a method of attachment by heat fusion involving spot welding or the like, a mechanical attachment method involving the use of bands or the like, and a method involving copper plating followed by solder attachment. The first-mentioned method of resin bonding has problems regarding heat resistance as well as durability over prolonged use. The heat fusion (welding) method is not appropriate for use with a magnetic amorphous member since the characteristics of amorphous materials are easily degraded by heat. As regards the third-mentioned mechanical attachment method, this is not capable of providing reliable attachment over the entire attachment surface. While the last-mentioned solder attachment method has the best potential, it is still inadequate as regards durability.